Experimental Search for the Smallest Stable Multiply Charged Anions in the Gas Phase

An extensive experimental search for the smallest stable gaseous multiply charged anions (MCAs) was pursued. Four penta-atomic dianions, PtX₄^2- and PdX₄^2- (X0ex0ex=0ex0exCl,Br), were found to be the smallest observable using an electrospray ion source. Photodetachment experiments revealed, however, that PtCl₄^2- and PtBr₄^2- are electronically metastable with negative electron-binding energies. PdCl₄^2- and PdBr₄^2- were found to be electronically stable, but thermodynamically unstable against fragmentation. Lifetimes were estimated using an ion-trap technique for the four observed metastable MCAs, providing further insight into their electronic and thermodynamic stability.